Everytime there's a bug for me, being English is the problem.
In the US, the spelling color is used to match the spelling of the word's Latin etymon, and to make all derivatives consistent (colorimeter, colorize, colorless, etc). Elsewhere in the English-speaking world, the spelling colour has been retained

I've broken the habit ... but as a kid in school, my English teacher used to give me crap about spelling it colour.

Now I'm a born an raised Californian, so I understood that's not how we do it ... but still happily pointed out it was a valid spelling and continued to do it to mess with him.

I just don't know where I picked up the habit in the first place ...